Open-end spinning of discrete fibers involves imparting twist to a body of discrete fibers to spin them into a yarn in a zone wherein the open end is the area where the discrete fibers are twisted initially. Previously known methods of open end or break spinning have been classified by the Shirley Institute in four groups. Two of these groups are vortex systems in which the open end of the yarn is twisted in a fluid vortex, usually air; and circumferential assembly systems in which fibers are conveyed in an airstream and are spun on a rapidly rotating cylindrical surface.
Examples of the latter type are disclosed, for example, in U.S. application Serial No. 782,310, filed by Dammann et al, on Mar. 28, 1977, now U.S. Pat. No. 4,130,983. That application also discloses that the rotating surfaces may be air permeable, symmetric or asymmetric hyperboloids of revolution.
The hyperboloids of revolution are arranged in such a way that their axes are located in planes parallel to each other or that each of them has a generating line which is parallel to the yarn formation line. The yarn formation line is the zone of the narrowest gap formed between the hyperboloids. Along said gap the air entrant nozzles of suction devices are arranged inside the hyperboloids of revolution. The hyperboloid surfaces terminate in a normal plane at each end. They may be asymmetrical in such a way that they do not have any plane of symmetry which is normal to their axes, whereby inadmissibly great tension forces are not exerted on the yarn being spun. The technical requirements are normally satisfied also by bodies of revolution which are cylindrical or constitute bodies of revolution of a parabola segment or any other curve segment which bulges outwardly from the axis of rotation. However, especially for the processing of polyester textile fibers (polyethylene terephthalate), the above mentioned spinning devices were afflicted by an uncertainty in operation, especially by the fact that the spinning results could not be reproduced with sufficient accuracy.